Continuous peripheral neural blockade to alleviate signs of experimentally induced severe forelimb pain in horses

Abstract

Objective: To investigate the efficacy and safety of a low-volume, single-catheter, continuous peripheral neural blockade (CPNB) technique to locally deliver bupivacaine to alleviate signs of severe forelimb pain resulting from experimentally induced tendonitis in horses.

Design: Randomized controlled experimental trial.

Sample: 14 horses and 5 forelimbs from equine cadavers.

Procedures: Horses underwent collagenase-induced superficial digital flexor tendonitis in the midmetacarpal region of 1 forelimb. To deliver analgesia, a closed-tip catheter was placed from lateral to medial, approximately 12 cm distal to the accessory carpal bone, between the suspensory ligament and accessory ligament of the deep digital flexor tendon. Success of catheter placement and anesthetic delivery was documented ex vivo in 5 forelimbs from equine cadavers. Effective analgesia in affected forelimbs of horses from continuous (n = 7) versus intermittent (7) local anesthetic delivery (intermittent peripheral neural blockade; IPNB) was compared over a 3-day period.

Results: Horses that received CPNB in the affected forelimb were less lame than horses that received IPNB. A lower proportion of CPNB-treated horses had behavioral and physiologic signs of pain, compared with IPNB-treated horses. Neither technique completely blocked the sensation of pain or resulted in swelling in the distal portion of the forelimb, vasodilation, or an increase in lameness. After removal, Staphylococcus aureus was cultured from 1 catheter tip.

Conclusions and clinical relevance: For short-term treatment, CPNB was more effective than IPNB for reduction in signs of severe pain in the distal aspect of the forelimb of horses.

Figure 1—

Photographic sequence of catheter placement for bupivacaine delivery to the medial and lateral palmar nerves. Through a vertical stab incision the introducer needle for catheter insertion is placed from lateral to medial, between the SL and ALDDFT approximately 12 cm distal to the accessory carpal bone (A). The catheter is inserted, and the introducer needle is carefully backed off the catheter (B). The introducer is placed in the subcutaneous plane from the dorsolateral aspect of the metacarpus to the stab incision and the proximal end of the catheter is placed retrograde through the introducer needle, to exit the skin at the dorsolateral aspect of the metacarpus (C), and is secured with a porous tape butterfly and skin suture. The proximal adapter for slip tip or luer lock attachment is placed on the catheter end (D). For continuous delivery of 0.5% bupivacaine, a battery-powered pump is filled (E) and secured to the distolateral aspect of the antebrachium (F and G) within the bandage (H).

Figure 2—

Photograph of a right forelimb in cross section from a cadaver horse. A closed-tip catheter was placed ex vivo approximately 12 cm distal to the accessory carpal bone, from lateral to medial, between the SL and ALDDFT. Seven milliliters of new methylene blue was injected, and the limb was frozen and sectioned in 1-cm increments. There is blue staining of the lateral palmar nerve (white arrow) and blue staining adjacent to the medial palmar nerve (gray arrow). DDFT = Deep digital flexor tendon. L = Lateral. M = Medial. MC2 = Second metacarpal bone. MC4 = Fourth metacarpal bone.

Figure 3—

Median (range) lameness scores (summary score from 3 blinded observers on 11-point scale; see Appendix) of 14 horses receiving CPNB (n = 7) or IPNB (7). Both groups received 10 mL of 0.5% bupivacaine injected over 3 days, and lameness was evaluated for 4 days after collagenase tendonitis induction. Individual analysis at each time point revealed significantly (*1-tailed P < 0.05) lower lameness score in CPNB-treated horses at t = 8 hours and t = 12 hours, compared with that of IPNB-treated horses. Error bars represent minimum and maximum scores.